I use Unity and Compiz on natty. I have Scale and Scale Addons enabled.

I set
CompizConfig Settings Manager > Scale > Bindings > Initiate Window Picker for window group to set my keyboard shortcut for "Initiate Window Picker for window group" to Super+grave (using grab key combo). If I open several gnome-terminal windows and hit Super+grave, nothing happens. Same for empathy, nautilus, gimp, and chromium. I've also tried binding to Super+a.

How does this feature work? What am I doing wrong?

I expect it to work like Application level Expose on a Mac where it lets you switch through all windows from a specific program (all Finder windows or all Chromium windows).

Meanwhile, in the bug report there is a workaround which works for me:

Prabhjot (prabhjotsbhatia) wrote on 2011-11-08: #8

A workaround:

Hi, I got it working using the "Commands" plugin and "DBus" plugin
enabled.
I use the following bash script as the command:
1. Go to command plugin in the Compiz config settings manager.
2. In one of the commands, enter this:

In bindings, bind that command to a screen edge or button or
keystroke, as you deem fit. Voila!! You have the Scale plugin for the
same application class

Note: you need Scale, DBus, Commands plugins enabled. You'd also need
to install awk if it isn't installed by default. This works one way
and to get out of the scale mode, press Esc or select a window.

For Ubuntu 12.04, add --reply-timeout=1 to the dbus-send command to avoid being stuck within scale.

for Ubuntu 12.04, add --reply-timeout=1 doesn't seem to work for me. It gets stuck everytime the command is called twice, which happen a lot when binded in a corner.
– user55822Aug 18 '12 at 15:26

Using ubuntu 17.04 and see this error -> Error org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.compiz was not provided by any .service files in /var/log/Xorg.0.log
– kapadDec 19 '17 at 11:02